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Irmgard Ziegler 《Die Naturwissenschaften》1987,74(12):563-572
Pteridines were originally described as pigments of insects and lower vertebrates. The electron-donating function of tetrahydrobiopterin for aromatic amino acid hydroxylation and thus, for neurotransmitter biosynthesis adduced the participation of unconjugated pterins in cellular metabolism. There has been increasing evidence moreover that they are signal molecules for intercellular recognition in primitive eucaryotes, as well as modifiers of signal polypeptides in higher vertebrates. 相似文献
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At three coastal dune sites at the island of Hiddensee, north-east Germany, vegetation cover was mapped during 2002 and compared
to vegetation surveys from the late 1980s and 1930s. Abiotic and biotic factors, which have been identified as being critical
for coastal dunes in former studies such as disturbance, salt spray or nutrient availability, were measured. Grazing and land-use
history were reviewed by literature and interviews. Tall graminoid communities, mainly Carex arenaria, are a common vegetation unit today. Development, distribution of these dominances and possible causes for its occurrence
have not been analysed. Generally, older successional vegetation units increased and pioneer stages decreased from the 1930s
until 2002. At the geologically youngest site, the southern dunes, grass encroachment by Carex arenaria was highest (ca. 50% cover in 2002), and age and density of trees lower than at the older, central dunes. Land-use changes
such as decrease in grazing pressure, additional feeding of livestock, increase in coastal protection measures and subsequent
decrease in shifting sands as well as varying availability of groundwater and amount of salt spray are relevant factors for
vegetation changes in coastal dunes over the past 70 years. Site-specific land-use differences such as livestock density and
land-use history have a stronger influence than atmospheric N-pollution on the vegetation composition of these acidic, coastal
dunes under low to moderate N-deposition loads of 6–8 kg N ha−1 yr−1. 相似文献
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Irmgard Flaschka C. W. Stockton W. R. Boggess 《Journal of the American Water Resources Association》1987,23(1):47-57
ABSTRACT: There is mounting evidence that increasing amounts of atmospheric carbon dioxide may lead to significant changes in global climate during the next century. The possible effects of such climatic changes on surface runoff in the Great Basin Region of the western United States has been investigated by applying water balance models to four watersheds in Nevada and Utah. The most probable change, a 2°C increase in average annual temperature coupled with a 10 percent decrease in precipitation, would reduce runoff from 17 to 28 percent of the present mean, with drier basins showing the greatest change. Decreasing precipitation by 25 percent causes runoff reductions of 33 to 51 percent. Equivalent changes to a cooler and wetter climate show corresponding increases in runoff of approximately the same magnitude, but such a shift is not considered likely. Based on projected water requirements for the year 2000, a change to a warmer and drier climate would cause severe water shortages in many parts of the Great Basin. 相似文献